A number of problems can arise in the horse’s foot as a result of having too much tension on the deep digital flexor (DDF) tendon.
These include differences in front feet appearance, clubbed feet, delamination of the hoof wall (white line disease), navicular inflammation, and laminitis (founder).
In many cases the cause of excessive DDF tension is unknown. Several factors have been implicated, such as developmental orthopedic disease (DOD), a difference in length between thoracic limbs, eating habits (i.e. standing with one limb forward and the other back), pain higher up in the affected limb(s), the horse being “right-” versus “left-handed” (or vice-versa), genetics, and rate of growth. Despite the actual cause(s), the consequences of increased DDF tension are fairly consistent.
Typically, one thoracic limb demonstrates more DDF tension than the other. This is usually evidenced by the horse having two different front feet (one more contracted than the other). In our experience, the RF limb appears to be more affected 70% of the time.
In order to understand the effects excessive DDF tendon tension has in the foot, it is important to first understand the anatomy. The thoracic deep digital flexor tendon arises from the deep digital flexor muscle. The DDF muscle has two portions, one attaching on the ulna and the other (larger) attaching to the humerus. The muscle courses along the backside of the radius, eventually giving rise to the DDF tendon just above the carpus (knee).
The DDF tendon courses behind the carpus, down along the back (palmar aspect) of the cannon bone, around the back of the fetlock, around the navicular bone in the back of the foot, and inserts on the underside of the third phalanx (P3).
The location, routing, and attachments of the DDF result in two primary forces that affect structures within the foot.
- Pressure across the navicular bone and
- A downward or rotational pull on P3.
1) Clubbed foot:A foot is “clubbed” if the heel has grown to the point of breaking the distal limb axis forward, where the angle of the foot is steeper than the angle of the pastern. Because the tendon inserts on the underside of P3, increased tension will pull or “rotate” the bone downward, eventually creating a steep hoof angle, high/contracted heels, and potentially a broken-forward axis. A broken-forward axis can in turn result in malarticulation of the coffin joint and arthritis. This can occur in one or both front feet.
It is important to realize that although one foot may be smaller, higher, and more con- tracted than the other it may not necessarily be “clubbed”. A difference in foot angle, however, does suggest that the “steeper” limb has more DDF tension than its contra- lateral counterpart. A foot is not considered “clubbed” until the distal limb axis is broken forward (A).
2) Delamination/ “white line” disease of the foot: Rotation of P3 downward can result in separation of the sensitive and insensitive tissue along the laminar interface of the hoof. This is called delamination. This separation occurs slowly and is usually not associated with inflammation and pain (laminitis). Dishing of the hoof wall generally confirms that separation/ delamination has occurred.
Many owners treat this problem by infusing a combination of astringents, antibiotics, and fungicides into the separated tissue in hopes that they will eliminate the various infectious agents that frequently invade this area. We should realize, however, that the primary problem is not infection. Bacteria and fungi are everpresent! The problem is the fact that they have an open space into which to invade. Eliminating the delaminated (open) space effectively eliminates the infection.
3) Navicular inflammation: The DDF tendon uses the navicular bone as a fulcrum as it courses around the back of the foot. The anatomy is similar to the way a pulley functions as a fulcrum for a rope. Increased tension on the DDF tendon results in increased pressure across the navicular bone and a greater chance of developing inflammation in this area. Simply put, this is why horses develop “navicular disease”.
4) Laminitis: Since laminitis can result from both biomechanical and metabolic processes within the laminae of the foot, increased tension/ distraction along the laminar interface (via the tension on the DDF tendon) increases the horse’s predisposition for developing laminitis as well as his/ her chances of experiencing digital collapse (P3 rotation) once tissue weakening has occurred.
Since the aforementioned problems can be directly linked to excessive tension of the DDF tendon, it stands to reason that treatment strategies should include techniques designed to reduce DDF tension. Less DDF tension means less pressure across the navicular bone and less rotational pull on P3.
Tension on the DDF tendon can be reduced by a) lengthening the tendon or b) shortening the distance between the origin and insertion of the tendon.
1) Trimming as much heel as possible. This will INCREASE DDF tension, but may also help to stretch t. This should not be done if the horse develops lameness or if there is evidence of hoof delamination (dishing). It should also not be attempted in older horses, as it will usually worsen the problem(s).
2) Massage of the DDF tendon muscle. “Loosening” the DDF muscle will reduce DDF tendon tension. We recommend consulting a massage therapist on effective ways to relax the DDF muscle.
3) Decreasing energy intake. Some people associate increased DDF tension with growth rate. If the bones grow faster than the tendons, tendon tension may increase to an excessive level.
4) Proper diet. Flexural contracture of the DDF tendon, one manifestation of developmental orthopedic disease (DOD), has been associated with mineral imbalance (e.g. copper deficiency).
5) Distal accessory (check) desmotomy. The distal accessory or “check” ligament is an extension of the palmar carpal ligament (on the back of the carpus) that joins the DDF tendon at the level of the mid cannon bone. By restricting excessive movement of the DDF tendon through it attachments, the distal accessory ligament functions to keep the DDF tendon in “check”. If excessive DDF tendon tension persists by the time the horse reaches 8-10 months of age, then surgical intervention in the form of distal accessory (check) desmotomy is often elected. Cutting the check ligament physically releases tension on the DDF tendon, allowing it to “lengthen”.
After 18-24 months or age, the DDF tendon has lost most of its pliability, and is generally not willing to “stretch” easily. In older horses, therefore, treatment strategies are usually directed at shortening the distance between the origin and insertion of the tendon. This is typically accomplished through corrective shoeing.
The two basic ways that DDF tension is reduced through corrective shoeing is by:
1) Increasing heel length relative to toe length. This can be accomplished by trimming more toe (from the solar or bottom surface) than heel. However, since only so much toe is available for trimming, the use of a wedged pad is often required. Elevating the heels results in “dropping” of the fetlock. Consequently, tension is transferred from the flexor tendons to the suspensory apparatus.
It should be noted, however, that there is a limit to the degree one can elevate the heels. Excessive heel elevation can result in malarticulation of the coffin joint, crushing of the heels, and/ or excessive stress to the suspensory apparatus.
2) Facilitating breakover of the foot. By moving the breakover point of the foot backward (palmarad), the stride length of the limb is shortened. Consequently, there is less degree of limb/ foot extension and therefore less tension on the DDF tendon during movement. The breakover point is moved backward by trimming toe and/or rolling or rockering the shoe.
When rolling the shoe, the farrier rasps or grinds the corner of the shoe where it meets the ground surface at the toe area. The 90° angle at this corner is eliminated and is therefore removed from the breakover process. When rockering the shoe, the farrier elevates the front of the shoe off of the ground, usually at about a 30° angle. The front corner of the shoe is completely lifted off of the ground and is therefore not incorporated in the breakover process. Rockering the shoe influences where along the toe the foot will break over, so it is important to know the foot’s normal breakover pattern.
The approach used to facilitate breakover is generally determined by farrier preference.
In cases of a severe and/or life-threatening foot problem occurring as a result of excessive DDF tension (i.e. laminitis), transection of the tendon may be considered. Of course, cutting the tendon eliminates its tension and therefore dramatically decreases the pressure across the navicular bone as well as the downward pull on the third phalanx. Consequently, this technique can quickly and effectively alleviate clinical problems associated with excessive DDF tension. However, since an intact DDF tendon is required for sound movement, this approach is considered only for salvage purposes; return to performance is not expected.
As previously stated, the original cause of excessive DDF tendon may not be known or treatable. Therefore, DDF tension may continue to increase over time, constantly challenging our efforts to alleviate it. No treatment strategy can therefore be considered a permanent “fix”.
If you have any further questions or concerns regarding problems related to excessive tension of the deep digital flexor tendon, don’t hesitate to call the office and let us know.